The formation of polyethylene into a hollow porous fiber or tube is described in various documents including: U.S. Pat. No. 3,423,491; U.S. Pat. No. 4,020,230; and the Technical Information Bulletin related to porous plastic tubes issued by A. T. Ramoto Plastics, Ltd., Israel (February, 1976), and in the specification of Japanese Patent Application Laid-Open No. 137026/77.
According to U.S. Pat. Nos. 3,423,491 and 4,020,230, porous hollow fibers are prepared by heating and melting a mixture consisting of polyethylene and a specific plasticizer, such as low-molecular weight ester, sulfolane, or polyglycol, forming the molten mixture into a hollow product, and thereafter extracting the plasticizer with a selective solvent which does not dissolve the polyethylene but does dissolve the plasticizer, so that the fibers will contain pores that penetrate therethrough. Namely, according to U.S. Pat. No. 3,423,491, porous hollow fibers which exhibit a de-salting of greater than 75% and which have properties suited for reverse osmosis, are obtained depending upon the conditions in which the plasticizer is extracted. According to U.S. Pat. No. 4,020,230, furthermore, there are produced porous polyethylene hollow fibers having a maximum pore radius of up to about 50 angstroms, which are capable of inhibiting the passage of more than 95% of albumin which has a radius of gyration about 30 angstroms.
U.S. Pat. No. 4,020,230 further teaches that it is not suitable to assess the diameters of effective micropores of the membrane which serves as a separating member relying upon a pore size observed by an electron microscope, but it is desirable to judge the pore size by a filtration experiment using a solution of a solute having a known molecular dimension, for instance an albumin solution.
In fact, although the distribution of pore size according to the above-mentioned invention is estimated to be from about 0.1 to 1 microns as observed through an electron microscope, a maximum diameter of the effective pores is less than 0.005 micron (50 angstroms) if measured by the filtration experiment. Accordingly, the distribution of pore size observed by the electron microscope does not exactly correspond to the function as a practical separating member.
Such a phenomenon is usually experienced in developing separating materials. That is to say, the distribution of micropores calculated relying upon a static means, such as mercury porosimeter or electron microscope, results in a value that substantially deviates from the diameters of effective pores of the separating materials. Therefore, in order to more correctly reflect the porous structure of the membrane and to practically evaluate the separating materials relying upon the structure of the porous membrane, it is highly desirable to measure the filtering characteristics of the membrane for a substance which has a known particle size.
On the other hand, the Technical Information Bulletin issued by A. T. Ramoto Plastics, Ltd. of Israel teaches a porous tube which has a large gas permeability, and which consists of a low-density polyethylene or a high-density polyethylene having an inner diameter of 1 to 12 mm and an average pore diameter of 1 to 30 microns. It is believed that the porous tube there described is produced by a process which is disclosed in the specification of U.S. Pat. No. 4,182,582 that was filed by A. T. Ramoto Plastics, Ltd. According to this patent, the polyethylenes and a pore-forming additive are extruded together, the pore-forming additive is extracted in a subsequent step, and fine pores are formed in the wall of the tube in much the same manner as U.S. Pat. Nos. 3,423,491 and 4,020,230 mentioned above. The tube is used chiefly for aerating gases into a liquid. Although there is a description related to the pore size in the Ramoto patent there is no description which deals with the permeability of a solute through the membrane.
The specification of Japanese Patent Application Laid-Open No. 137026/77 describes certain hollow fibers having an average pore diameter of 0.01 to 0.5 microns, that are obtained by melt-spinning a thermoplastic crystalline polymer and that the fibers are oriented in the lengthwise direction. In Example 3 of this document, the only example dealing with polyethylene, the pore diameter ranges from 0.02 to 0.2 micron (measured presumably by the method of mercury porosimetry), and the porosity is 23% at the most. However, there is no description which teaches the permeability of the solute.
Thus the various pore sizes taught in the prior art, i.e., U.S. Pat. No. 4,182,582, does not present practical effective pore size of membrane for separating the solute as disclosed in the specification of U.S. Pat. No. 4,020,230.
As will be apparent from the above-mentioned prior art, the porous hollow fibers composed of polyethylene can be produced by a variety of methods. However, polyethylene hollow fibers have not yet been produced suitable for use as a membrane for fractionating blood components, which membranes exhibit excellent albumin permeability, have a large porosity, have substantial water permeability, and which contain fine pores that penetrate therethrough.
In particular, porous hollow fibers that are obtained from polyethylene without the need to use a solvent or a plasticizer and that have micropores suited for separating blood components, serve as a clean membrane without containing residual solvent or plasticizer that comtaminates blood components being permeated. Further, such porous hollow fibers when employed in an apparatus for separating blood, presents an increased membrane area per unit of apparatus and minimizes the amount of blood that may remain in the apparatus. In this case, the hydrophobic property of the polyethylene does not at all hinder the purpose to which the membrance is put. The hydrophobic property can be suitably converted into a hydrophilic property through treatment with alcohol, an oxidation treatment with ozone, or through any other conventionally known physical and chemical treatment to achieve the characteristic desired.